Self aligned method of forming a semiconductor memory array of floating gate memory cells with horizontally oriented edges

ABSTRACT

A self aligned method of forming an array of floating gate memory cells, and an array formed thereby, wherein each memory cell includes a trench formed into a surface of a semiconductor substrate, and spaced apart source and drain regions with a channel region formed therebetween. The drain region is formed underneath the trench. An electrically conductive floating gate is formed over and insulated from a portion of the channel region, with a horizontally oriented edge extending therefrom. An electrically conductive control gate is formed having a first portion disposed in the trench and a second portion disposed adjacent to and insulated from the floating gate edge.

[0001] This application claims the benefit of U.S. ProvisionalApplication No. 60/343,634, filed Dec. 27, 2001, and entitled A SuperSelf-Aligned Flash E2PROM With Vertical Word-Line Transistor For Programand Horizontal-Oriented Floating-Gate Tips For Erase, and of U.S.Provisional Application No. 60/355,363, filed Feb. 6, 2002, and entitledA Super Self-Aligned Flash E2PROM With Vertical Word-Line Transistor ForProgram and Horizontal-Oriented Floating-Gate Tips For Erase—SAC Optionand Metal Source-Line Option.

TECHNICAL FIELD

[0002] The present invention relates to a self-aligned method of forminga semiconductor memory array of floating gate memory cells. The presentinvention also relates to a semiconductor memory array of floating gatememory cells of the foregoing type.

BACKGROUND OF THE INVENTION

[0003] Non-volatile semiconductor memory cells using a floating gate tostore charges thereon and memory arrays of such non-volatile memorycells formed in a semiconductor substrate are well known in the art.Typically, such floating gate memory cells have been of the split gatetype, or stacked gate type.

[0004] One of the problems facing the manufacturability of semiconductorfloating gate memory cell arrays has been the alignment of the variouscomponents such as source, drain, control gate, and floating gate. Asthe design rule of integration of semiconductor processing decreases,reducing the smallest lithographic feature, the need for precisealignment becomes more critical. Alignment of various parts alsodetermines the yield of the manufacturing of the semiconductor products.

[0005] Self-alignment is well known in the art. Self-alignment refers tothe act of processing one or more steps involving one or more materialssuch that the features are automatically aligned with respect to oneanother in that step processing. Accordingly, the present invention usesthe technique of self-alignment to achieve the manufacturing of asemiconductor memory array of the floating gate memory cell type.

[0006] There is a constant need to shrink the size of the memory cellarrays in order to maximize the number of memory cells on a singlewafer. It is well known that forming memory cells in pairs, with eachpair sharing a single source region, and with adjacent pairs of cellssharing a common drain region, reduces the size of the memory cellarray. However, a large area of the array is typically reserved for thebit-line connection to the drain regions. The bit-line area is oftenoccupied by the contact openings between memory cell pairs, and thecontact to wordline spacing (which strongly depends upon lithographygeneration), contact alignment and contact integrity. In addition,significant space is reserved for the word-line transistor, the size ofwhich is set by lithography generation and junction scaling.

[0007] Another aspect addressed by the present invention involves theerase performance of the memory cell. FIG. 1 illustrates a well knownnon-volatile memory cell design, which includes a floating gate 1disposed over and insulated from a semiconductor substrate 2 havingsource and drain regions 3/4. A control gate 5 has a first portion thatis disposed laterally adjacent to the floating gate 1, and a secondportion that is disposed vertically over and overlapping the floatinggate 1. The floating gate 1 includes a relatively sharp edge 6 thatextends upwardly toward the control gate second portion. The edge 6extending toward the overlapping portion of the control gate 5 enhancesFowler-Nordheim tunneling used to erase the memory cell. As the cellsize is scaled down, at least some of the overlap between control gate 5and floating gate 1 must be maintained so that the upwardly orientedpointed edges can be used for the erase function. This cell architectureimposes a scaling limit on the erase coupling ratio due to the finiteoverlap capacitance between the control gate 5 and the floating gate 1.

[0008] There is a need for a non-volatile, floating gate type memorycell array with significant cell size reduction without adverselycompromising the erase coupling ratio of the memory cell.

SUMMARY OF THE INVENTION

[0009] The present invention solves the above mentioned problems byproviding a self aligned method of forming memory cells with reducedsize, by minimizing the space needed for the bit-line connection andword-line transistor and eliminating the need for a vertical overlapbetween the control gate and floating gate, and a memory cell arrayformed thereby.

[0010] The present invention is an electrically programmable anderasable memory device that includes a substrate of semiconductormaterial of a first conductivity type, first and second spaced-apartregions formed in the substrate and having a second conductivity typewith a channel region of the substrate defined therebetween, anelectrically conductive floating gate disposed over and insulated fromat least a portion of the channel region, wherein the floating gateincludes a horizontally oriented edge extending from a lateral side ofthe floating gate, and an electrically conductive control gate having atleast a portion thereof disposed laterally adjacent to and insulatedfrom the horizontally oriented edge.

[0011] In another aspect of the present invention, and array ofelectrically programmable and erasable memory devices includes asubstrate of semiconductor material of a first conductivity type, andspaced apart isolation regions formed on the substrate which aregenerally parallel to one another and extend in a first direction, withan active region between each pair of adjacent isolation regions. Eachof the active regions includes a plurality of memory cells, where eachof the memory cells includes first and second spaced-apart regionsformed in the substrate having a second conductivity type with a channelregion of the substrate defined therebetween, an electrically conductivefloating gate disposed over and insulated from at least a portion of thechannel region wherein the floating gate includes a horizontallyoriented edge extending from a lateral side of the floating gate, and anelectrically conductive control gate having at least a portion thereofdisposed laterally adjacent to and insulated from the horizontallyoriented edge.

[0012] In yet another aspect of the present invention, a method offorming a semiconductor memory cell includes the steps of forming firstand second spaced apart regions in a semiconductor substrate having aconductivity type different from that of the substrate, wherein achannel region of the substrate is defined between the first and secondregions, forming a floating gate of electrically conductive materialdisposed over and insulated from at least a portion of the channelregion, wherein the floating gate includes a horizontally oriented edgeextending from a lateral side of the floating gate, and forming acontrol gate of electrically conductive material having at least aportion thereof disposed laterally adjacent to and insulated from thehorizontally oriented edge.

[0013] In still yet another aspect of the present invention, a method offorming an array of semiconductor memory cells includes the steps offorming spaced apart isolation regions on the substrate having a firstconductivity type which are generally parallel to one another and extendin a first direction, with an active region between each pair ofadjacent isolation regions, forming a plurality of spaced apart firstand second regions in the semiconductor substrate having a secondconductivity type, wherein a plurality of channel regions in the activeregions of the substrate are defined each extending between one of thefirst regions and one of the second regions, forming a plurality offloating gates of electrically conductive material each disposed overand insulated from at least a portion of one of the channel regions,wherein each of the floating gates includes a horizontally oriented edgeextending from a lateral side of the floating gate, and forming aplurality of electrically conductive control gates each having at leasta portion thereof disposed laterally adjacent to and insulated from oneof the horizontally oriented edges.

[0014] In even another aspect of present invention, a method ofoperating an electrically programmable and erasable memory device havingan electrically conductive floating gate disposed over and insulatedfrom a substrate of semiconductor material, and an electricallyconductive control gate having at least a portion thereof disposedlaterally adjacent to the floating gate and insulated therefrom by aninsulating material, includes the step of placing a voltage on thecontrol gate that is sufficiently positive relative to a voltage of thefloating gate to induce electrons on the floating gate to laterallytunnel from a horizontally oriented edge extending from a lateral sideof the floating gate, through the insulating material, and onto thecontrol gate via Fowler-Nordheim tunneling.

[0015] And in one more aspect of the present invention, a method ofoperating an electrically programmable and erasable non-volatile memorycell having a first and a second state, and including an electricallyconductive floating gate disposed over and insulated from a substrate ofsemiconductor material, and an electrically conductive control gatehaving at least a portion thereof disposed laterally adjacent to thefloating gate, includes the steps of establishing a first state of thememory cell by injecting electrons from a drain region of the substrateonto the floating gate, wherein the source region is disposed below asurface of the substrate and the injected electrons travel through thesubstrate in a direction generally perpendicular to the surface of thesubstrate, and establishing a second state of the memory cell byremoving electrons from the floating gate to the control gate viaFowler-Nordheim tunneling through an insulating material disposedtherebetween, wherein the removed electrons tunnel from a horizontallyoriented edge extending from a lateral side of the floating gate,through the insulating material, and onto the control gate in adirection generally parallel to the surface of the substrate.

[0016] Other objects and features of the present invention will becomeapparent by a review of the specification, claims and appended figures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0017]FIG. 1 is a side cross-sectional view showing a conventionalsplit-gate non-volatile memory cell.

[0018]FIG. 2A is a top view of a semiconductor substrate used in thefirst step of the method of present invention to form isolation regions.

[0019]FIG. 2B is a cross sectional view of the structure taken along theline 1-1 showing the initial processing steps of the present invention.

[0020]FIG. 2C is a top view of the structure showing the next step inthe processing of the structure of FIG. 2B, in which isolation regionsare formed.

[0021]FIG. 2D is a cross sectional view of the structure in FIG. 2Ctaken along the line 1-1 showing the isolation stripes formed in thestructure.

[0022]FIG. 2E is a cross sectional view of the structure in FIG. 2Ctaken along the line 1-1 showing the two types of isolation regions thatcan be formed in the semiconductor substrate: LOCOS or shallow trench.

[0023]FIGS. 3A-3U are cross sectional views taken along the line 2-2 ofFIG. 2C showing in sequence the next step(s) in the processing of thestructure shown in FIG. 2C, in the formation of a non volatile memoryarray of floating memory cells according to the present invention.

[0024]FIGS. 4A-4G are cross sectional views of a semiconductor structureshowing in sequence the steps in a first alternate processing of thesemiconductor structure of FIG. 3O in the formation of a non volatilememory array of floating memory cells of the present invention.

[0025]FIGS. 5A-5I are cross sectional views of a semiconductor structureshowing in sequence the steps in a second alternate processing of thesemiconductor structure of FIG. 3M in the formation of a non volatilememory array of floating memory cells of the present invention.

[0026]FIGS. 6A-6J are cross sectional views of a semiconductor structureshowing in sequence the steps in a third alternate processing of thesemiconductor structure of FIG. 3M in the formation of a non volatilememory array of floating memory cells of the present invention.

[0027]FIGS. 7A-7Z are cross sectional views of a semiconductor structureshowing in sequence the steps in a fourth alternate processing of thesemiconductor structure of FIG. 3C in the formation of a non volatilememory array of floating memory cells of the present invention.

[0028]FIGS. 8A-8W are cross sectional views of a semiconductor structureshowing in sequence the steps in a fifth alternate processing of thesemiconductor structure of FIG. 7F in the formation of a non volatilememory array of floating memory cells of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0029] The present invention is a method of forming columns ofnon-volatile memory cells in active regions that are separated byisolations regions, and an array of memory cells formed thereby. Thememory cells utilize a unique memory cell design that allows significantcell size reduction without adversely compromising the erase couplingratio of the memory cell.

[0030] Isolation Region Formation

[0031] Referring to FIG. 2A there is shown a top plan view of asemiconductor substrate 10 (or a semiconductor well), which ispreferably of P type and is well known in the art. A layer of insulatingmaterial 12, such as silicon dioxide (hereinafter “oxide”), is depositedthereon as shown in FIG. 2B. The insulating layer 12 is formed on thesubstrate 10 by well known techniques such as oxidation or deposition(e.g. chemical vapor deposition or CVD), forming a layer of oxidepreferably 80 Å thick. A layer of polysilicon 14 (hereinafter “poly”) isdeposited on top of the layer of insulating material 12 (e.g. 200 to 700Å thick). The deposition and formation of the polysilicon layer 14 onthe insulating layer 12 can be made by a well known process such as LowPressure CVD or LPCVD. This poly layer 14 can be doped by ionimplantation or in-Situ doped. A silicon nitride layer 18 (hereinafter“nitride”) is deposited over the polysilicon layer 14, preferably by CVD(e.g. 500-1000 Å thick). This nitride layer 18 is used to define theactive regions during isolation formation. Of course, all of theforgoing described parameters and the parameters described hereinafter,depend upon the design rules and the process technology generation. Whatis described herein is for a 0.13 micron process. However, it will beunderstood by those skilled in the art that the present invention is notlimited to any specific process technology generation, nor to anyspecific value in any of the process parameters described hereinafter.

[0032] Once the insulating layer 12, the polysilicon layer 14, and thesilicon nitride 18 have been formed, suitable photo-resistant material19 is applied on the silicon nitride layer 18 and a masking step isperformed to selectively remove the photo-resistant material fromcertain regions (stripes 16). Where the photo-resist material 19 isremoved, the silicon nitride 18, the polysilicon 14 and the underlyinginsulating material 12 are etched away in stripes 16 formed in the Y orcolumn direction, as shown in FIG. 2C, using standard etching techniques(i.e. anisotropic etch processes). The distance W between adjacentstripes 16 can be as small as the smallest lithographic feature of theprocess used. Where the photo resist 19 is not removed, the siliconnitride 18, the polysilicon region 14 and the underlying insulatingregion 12 are maintained. The resulting structure is illustrated in FIG.2D, with active regions 17 interlaced with isolation regions 16. As willbe described, there are two embodiments in the formation of theisolation regions: LOCOS and STI. In the STI embodiment, the etchingcontinues into the substrate 10 to a predetermined depth.

[0033] The structure is further processed to remove the remaining photoresist 19. Then, an isolation material 20 a or 20 b, such as silicondioxide, is formed in the regions or “grooves” 16. The nitride layer 18is then selectively removed to form the structure shown in FIG. 2E. Theisolation can be formed via the well known LOCOS process resulting inthe local field oxide 20 a (e.g. by oxidizing the exposed substrate), orit can be formed via a shallow trench process (STI) resulting insilicon-dioxide being formed in the region 20 b (e.g. by depositing anoxide layer, followed by a Chemical-Mechanical-Polishing (CMP) etch). Itshould be noted that during the LOCOS formation, a spacer may benecessary to protect the side walls of poly layer 14 during theformation of the local field oxide.

[0034] The remaining polysilicon layer 14 and the underlying insulatingmaterial 12 form the active regions. Thus, at this point, the substrate10 has alternating stripes of active regions and isolation regions withthe isolation regions being formed of either LOCOS insulating material20 a or shallow trench insulating material 20 b. Although FIG. 2E showsthe formation of both a LOCOS region 20 a and a shallow trench region 20b, only one of the LOCOS process (20 a) or the shallow trench process(20 b) will be used. In the preferred embodiment, the shallow trench ofinsulating material 20 b will be formed, because it can be moreprecisely formed at smaller design rules. The structure in FIG. 2Erepresents a self aligned structure, which is more compact than astructure formed by a non self-aligned method.

[0035] A non self-aligned method of forming the structure shown in FIG.2E, which is well known and is conventional, is as follows. Regions ofisolation 20 are first formed in the substrate 10. This can be done bydepositing a layer of silicon nitride on the substrate 10, depositingphoto-resist, patterning the silicon nitride using a first masking stepto expose selective portions of the substrate 10, and then oxidizing theexposed substrate 10 using either the LOCOS process or the STI processwhere silicon trench formation and trench fill are involved. Thereafter,the silicon nitride is removed, and the layer of silicon dioxide 12 (toform the gate oxide) is deposited over the substrate 10. The layer ofpolysilicon 14 is deposited over the gate oxide 12. The layer ofpolysilicon 14 is then patterned using a second masking step andselective portions removed. Thus, the polysilicon 14 is not self alignedwith the regions of isolation 20, and a second masking step is required.Further, the additional masking step requires that the dimensions of thepolysilicon 14 have an alignment tolerance with respect to the regionsof isolation 20. It should be noted that the non self-aligned methoddoes not utilize nitride layer 18. The memory array of the presentinvention can be made by either the self-aligned process or the nonself-aligned process.

[0036] Memory Array Formation

[0037] With the structure shown in FIG. 2E made using either the selfaligned method or the non self-aligned method, the structure is furtherprocessed as follows. FIGS. 3A to 3U show the cross section of theactive region structure 17 from a view orthogonal to that of FIGS. 2Band 2E, as the next steps in the process of the present invention areperformed. It should be appreciated that while only a small segment ofactive region 17 is shown, the processing steps illustrated below forman array of such regions.

[0038] An insulating layer 22 is first formed on the structure.Specifically, an oxide layer 22 is formed over poly layer 14 (e.g. 100to 200 Å thick). A thick poly layer 24 is then formed over the oxidelayer 22 (e.g. ˜3000 Å thick). The resulting active region structure isshown in FIG. 3A.

[0039] A conventional photo-lithography (masking) scheme is next used toform semi-recessed first trenches 26 in the structure in the followingmanner. A suitable photo-resistant material is applied on the poly layer24 and a masking step is performed to selectively remove thephoto-resistant material from selected parallel stripe regions (underwhich the first trenches 26 will be formed). Where the photo-resistmaterial is removed, the exposed polysilicon 24 is removed using astandard poly etch process (e.g. anisotropic poly etch using oxide layer22 as an etch stop), leaving blocks of polysilicon 28 with firsttrenches 26 formed therebetween. The width of poly blocks 28 can be assmall as the smallest lithographic feature of the process used. Theremaining photo resist is removed, resulting in the structureillustrated in FIG. 3B.

[0040] A nitride deposition step is used to form a nitride layer 30(e.g. 300 to 500 Å thick) over the structure, as illustrated in FIG. 3C.A thick oxide layer 32 (e.g. 2500 Å thick) is then deposited overnitride layer 30, as illustrated in FIG. 3D. A planarization etchprocess follows (e.g. Chemical-Mechanical-Polishing (CMP) etch), whichetches the thick oxide layer 32 down even with the tops of poly blocks28 (using poly blocks 28 as the polishing stopper). The CMP etch alsoremoves the portions of nitride layer 30 over poly blocks 28. Theresulting structure is shown in FIG. 3E.

[0041] A poly etch process is used to remove poly blocks 28 (using oxidelayer 22 as an etch stop) to form second trenches 34. A controlled oxideetch is used to remove the portions of oxide layer 22 exposed at thebottoms of second trenches 34 (using poly layer 14 as an etch stop).This oxide etch also consumes some of thick oxide layer 32. Another polyetch process is performed to remove portions of poly layer 14 exposed atthe bottoms of the second trenches 34 (using oxide layer 12 as an etchstop). The resulting structure is shown in FIG. 3F.

[0042] An oxide sidewall layer 36 is formed on the exposed ends of polylayer 14 at trenches 34 using a thermal oxidation process. Suitable ionimplantation is then made across the entire surface of the structure.Where the ions have sufficient energy to penetrate exposed portions ofoxide layer 12 in each second trench 34, they then form first regions(source regions) 38 in the substrate 10 having a conductivity type (e.g.N type) that is different from that of the surrounding substrate (e.g. Ptype). In all other regions, the ions are absorbed by the underlyingstructure, where they have no effect. The implanted source regions 38are self aligned to the second trenches 34. Sidewall spacers 40 are thenformed against the walls of the second trenches 34. Formation of spacersis well known in the art, and involves the deposition of a material overthe contour of a structure, followed by an anisotropic etch process,whereby the material is removed from horizontal surfaces of thestructure, while the material remains largely intact on verticallyoriented surfaces of the structure. Spacers 40 can be formed of anydielectric material with good etch selectivity to nitride layer 30 andto substrate silicon 10. In the preferred embodiment, insulating spacers40 are formed of oxide by depositing a thin layer of oxide (e.g. 200 Å)over the entire structure, followed by an anisotropic etch process, suchas the well known Reactive Ion Etch (RIE dry etch), to remove thedeposited oxide layer except for spacers 40. This oxide etch processalso removes the exposed portions of oxide layer 12 at the bottom ofsecond trenches 34 to expose the substrate 10, as well as some of oxidelayer 32. The resulting structure is shown in FIG. 3G.

[0043] The second trenches are filled with poly blocks 42, preferably inthe following manner. A thick layer of poly silicon is deposited overthe structure. A planarization process (i.e. CMP) is used to remove thedeposited poly silicon except for poly blocks 42 in second trenches 34.An optional poly etch may be performed to slightly recess the uppersurfaces of poly blocks 34 relative to nitride layer 30 and oxide layer32, to address any topography issues, if present, that canunintentionally short poly blocks 42 to one another. The poly blocks 42run across the array columns (along line 1-1 in FIG. 2C) and are indirect electrical contact with source regions 38 of substrate 10. Theresulting structure is shown in FIG. 3H.

[0044] An oxide etch is used to remove oxide layer 32, leavingsemi-recessed third trenches 44 disposed between poly blocks 42. Anoxide layer 43 is then formed over the structure (e.g. 200-300 Å) byconventional techniques (e.g. LPCVD). The resulting structure isillustrated in FIG. 3I. Nitride spacers 46 are formed against thesidewalls of third trenches 44 by depositing a thick layer of nitride(e.g. 2000 Å), followed by an anisotropic nitride (RIE dry) etch toremove the deposited nitride except for spacers 46 (e.g. spacers havinga width of ˜1500 Å). The resulting structure is shown in FIG. 3J.

[0045] An oxide etch is next used to remove the exposed portions ofoxide layer 43 (i.e. those portions not protected by spacers 46). Thestructure is then subjected to a thermal oxidation process, which formsan oxide layer 48 over poly blocks 42. Oxide layer 48 is self aligned tothe poly blocks 42 (with a thickness, for example, of ˜600 Å). Duringthe thermal cycle of this oxidation process, the source regions 38 aredriven deeper into the substrate. The resulting structure is shown inFIG. 3K.

[0046] An anisotropic (dry) nitride etch is used to remove the portionsof nitride layer 30 that are exposed in third trenches (between nitridespacers 46), using oxide layer 22 as an etch stop. This nitride etchalso removes some of the exposed portions of nitride layer 30 adjacentpoly blocks 42 and spacers 46. An anisotropic oxide etch follows toremove the portions of oxide layer 22 that are exposed in third trenches44 (between nitride spacers 46), using poly layer 14 as an etch stop.This oxide etch also slightly consumes small portions of oxide layer 48.The resulting structure is shown in FIG. 3L.

[0047] An anisotropic (dry) poly etch is next performed to remove theportions of poly layer 14 that are exposed in third trenches (betweennitride spacers 46), using oxide layer 12 as an etch stop. Ananisotropic (dry) oxide etch follows, which removes the portions ofoxide layer 12 that are exposed in third trenches (between nitridespacers 46), which exposes the substrate 10 at the bottom of thirdtrenches 44. This oxide etch also slightly consumes small portions ofoxide layer 48. The resulting structure is shown in FIG. 3M.

[0048] Next, with the silicon substrate 10 left exposed at the bottom ofthird trenches 44, a silicon etch process is performed to extend thirdtrenches 44 down into substrate 10 (e.g. to a depth of 500 Å to 1500 Åbelow the substrate surface). Then, an oxide etch is used to removeoxide layer 48, and to etch the top of isolation oxide 20 a or 20 b(shown in FIG. 2E) down to a level generally equal to the bottom oftrenches 44. Thus, third trenches 44 are continuously formed and extendacross the active and isolation regions 17/16. Suitable ion implantationis once again made across the entire surface of the structure. The ionimplantation forms second regions 50 (buried bit-line drain regions) inthe active regions of substrate 10 underneath third trenches 44. The ionimplant also has the effect of doping (or further doping) the exposedpoly blocks 42. Outside of third trenches 44, and except for poly blocks42, the ions are blocked and have no effect. The resulting structure isshown in FIG. 3N. The second regions 50 are not formed under theportions of third trenches that are in the isolation regions, becausethe ions are blocked by the isolation oxide 20 a or 20 b.

[0049] An oxidation process is then performed, which oxidizes theexposed silicon surfaces of third trenches 44 to form a thin layer ofoxide 52 lining these silicon surfaces. This oxidation process alsooxidizes the exposed lateral sides 14 a of poly layer 14 that form partof the sidewalls of upper portions of third trenches 44, which resultsin horizontally oriented edges 54 each of which points directly towardone of the sidewalls of third trenches 44, as illustrated in FIG. 3O.Edges 54 can be elongated edges (i.e. like the sharp edge of a razorblade), or shorter edges (i.e. like the tip of a pencil). This oxidationprocess also re-grows the oxide layer 48 over poly blocks 42.

[0050] The formation of poly spacers 56 along sidewalls of thirdtrenches 44 follows, which is done by first depositing a poly layer overthe structure (e.g. 2000 Å thick). A dry anisotropic dry etch process isthen used to remove the deposited poly layer, except for poly spacers 56left along the sidewalls of third trenches 44. The resulting structureis shown in FIG. 3P.

[0051] A nitride etch process is then used to remove nitride spacers 46.An oxide etch follows to remove oxide layers 48 and 43, and the exposedportions of oxide layer 52 at the bottom of third trenches 44(in-between poly spacers 56), as shown in FIG. 3Q. A nitride etchprocess follows (e.g. in hot phosphoric acid), which removes nitridelayer 30, as shown in FIG. 3R. Insulating spacers 58 (preferably oxide)are formed along the bottoms of the third trench sidewalls by depositinga layer of oxide over the structure (e.g. 500 to 1200 Å thick), followedby an oxide etch (e.g. RIE). This oxide deposition and etch process alsoforms oxide spacers 60 adjacent poly spacers 56 (over poly edges 54),and oxide spacers 62 adjacent oxide spacers 40. The resulting structureis shown in FIG. 3S.

[0052] A metal deposition step is then performed, to deposit a metalsuch as tungsten, cobalt, titanium, nickel, platinum, or molybdenum overthe structure. The structure is then annealed, permitting the hot metalto flow and to seep into the exposed portions of the substrate 10 at thebottom of third trenches 44 to form a conductive layer of metalizedsilicon 64 (silicide) between oxide spacers 58. Metalized siliconregions 64 on substrate 10 can be called self aligned silicide (i.e.salicide), because they are self aligned to the second regions 50 byspacers 58. The hot metal also forms a conductive layer of metalizedpolysilicon 66 (polycide) on the exposed top portions of poly spacers56, and polycide layer 68 on the exposed top portions of poly blocks 42.The metal deposited on the remaining structure is removed by a metaletch process. The resulting structure is shown in FIG. 3T.

[0053] Passivation, such as BPSG 70, is used to cover the structure,including filling third trenches 44. A masking step is performed todefine etching areas over the salicide regions 64. The BPSG 70 isselectively etched in the etching areas to create contact openings thatare ideally centered over the salicide regions 64 (and second regions50). The contact openings are then filled with conductor metal by metaldeposition and planarizing etch-back, forming conductor contacts 72. Thesalicide layer 64 enhances conduction between the conductors 72 and thesecond regions 50. Polycide layers 66/68 enhance conduction along thelength of poly spacers 56 and poly blocks 42. A bit line connector 74 isadded to each active region by metal masking over the BPSG 70 to connecttogether all the contacts 72 in that active region. The final structureis shown in FIG. 3U.

[0054] As shown in FIG. 3U, first and second regions 38/50 form thesource and drain for each cell (although those skilled in the art knowthat source and drain can be switched during operation). Poly layer 14constitutes the floating gate and poly spacer 56 constitutes the controlgate for each of the memory cells. A channel region 76 for each memorycell is the surface portion of the substrate that is in-between thesource and drain 38/50. Each channel region 76 has two portions joinedtogether at a right angle: a vertical portion 78 extending along thevertical wall of third trench 44, and a horizontal portion 80 extendingbetween the third trench 44 and the source region 38. Each floating gate14 is disposed over, but insulated from, the horizontal portion 80 ofthe channel region 76, and a portion of the source region 38. Thefloating gates 14 each have the horizontally oriented edge 54 thatdirectly faces one of the control gates 56, but are insulated therefromby oxide layer 52. The process of the present invention forms pairs ofmemory cells that mirror each other, with a memory cell formed on eachside of the third trenches 44 that share a common bit-line region 50.Similarly, each source line region 38, which is in electrical contactwith poly block 42, is shared between adjacent memory cells fromdifferent mirror sets of memory cells. Each control gate 56, sourceregion 38 (with poly block 42 electrically connected thereto) and drainregion 50 continuously extend across the isolation regions 16 and activeregions 17, linking together one memory cell from each of the activeregions 17.

[0055] Memory Cell Operation

[0056] The operation of the memory cells will now be described belowwith reference to FIG. 3U. The operation and theory of operation of suchmemory cells are also described in U.S. Pat. No 5,572,054, whosedisclosure is incorporated herein by reference with regard to theoperation and theory of operation of erasing and reading a non-volatilememory cell having a floating gate and a control gate, floating gate tocontrol gate tunneling, and an array of such memory cells.

[0057] To initially erase a selected memory cell in any given activeregion 17, a ground potential is applied to both its source 38 and drain50. A high-positive voltage (e.g. +5-10 volts), is applied to thecontrol gate 56. Electrons on the floating gate 14 are induced throughthe Fowler-Nordheim tunneling mechanism to tunnel through the oxidelayer 52 to the control gate 56, leaving the floating gate 14 positivelycharged. Tunneling is enhanced by the horizontally oriented edge 54formed on each floating gate 14. It should be noted that since eachcontrol gate 56 continuously extends across the active regions 17 andisolation regions 16, one memory cell from each active region is erasedwhen the high voltage is applied to one of the control gates 56.

[0058] When a selected memory cell is desired to be programmed, a smallvoltage (e.g. 0.5 to 1.0 V) is applied to its drain region 50. Apositive voltage level in the vicinity of the threshold voltage of theMOS structure defined by the control gate 56 (on the order ofapproximately +1.8 volts) is applied to its control gate 56. A positivehigh voltage, on the order of 9 or 10 volts, is applied to its sourceregion 38. Electrons generated by the drain region 50 will flow from thedrain region 50 towards the source region 38 through the weakly-invertedvertical portion 78 of the channel region 76. As the electrons reach thehorizontal portion 80 of the channel region 76, they will see the highpotential of the near end of floating gate 14 (because the floating gate14 is more strongly capacitively coupled to the positively chargedsource region 38 than to the control gate 56). The electrons willaccelerate and become heated, with most of them being injected into andthrough the insulating layer 12 and onto the floating gate 14. Groundpotential and Vdd (approximately 1.5 to 3.3 volts depending upon thepower supply voltage of the device) are applied to the source lines 38and bit-line regions 50, respectively, for memory cell rows and columnsnot containing the selected memory cell. Ground potential is applied tothe control gates 56 for memory cell rows not containing the selectedmemory cell. Thus, only the memory cell in the selected row and columnis programmed.

[0059] The injection of electrons onto the floating gate 14 willcontinue until the reduction of the charge on the near end of thefloating gate 14 can no longer sustain a high surface potential alongthe horizontal channel region portion 80 to generate hot electrons. Atthat point, the reduced charge on the floating gate 14 will decrease theelectron flow from the drain region 50 onto the floating gate 14.

[0060] Finally, to read a selected memory cell, ground potential isapplied to its source region 38. A read voltage of approximately +1 voltis applied to its drain region 50 and approximately 1.5 to 3.3 volts(depending upon the power supply voltage of the device) is applied toits control gate 56. If the floating gate 14 is positively charged (i.e.the floating gate is discharged of electrons), then the horizontalportion 80 of the channel region 76 (directly underneath the floatinggate 14) is turned on. When the control gate 56 is raised to the readpotential, the vertical portion 78 of the channel region 76 (directlyadjacent the control gate 56) is also turned on. Thus, the entirechannel region 76 will be turned on, causing electrical current to flowfrom the source region 38 to the drain region 50. This would be the “1”state.

[0061] On the other hand, if the floating gate 14 is negatively charged,the horizontal portion 80 of the channel region 76 (directly below thefloating gate 14) is either weakly turned on or is entirely shut off.Even when the control gate 56 and the drain region 50 are raised to theread potential, little or no current will flow through horizontalportion 80 of channel region 76. In this case, either the current isvery small compared to that of the “1” state or there is no current atall. In this manner, the memory cell is sensed to be programmed at the“0” state. Ground potential is applied to the source lines 38, bit-lineregions 50, and control gates 56 for non-selected columns and rows soonly the selected memory cell is read.

[0062] The memory cell array includes peripheral circuitry includingconventional row address decoding circuitry, column address decodingcircuitry, sense amplifier circuitry, output buffer circuitry and inputbuffer circuitry, which are well known in the art.

[0063] The memory cell architecture of the present invention isadvantageous because it does not employ a finite, vertical overlap areabetween an upwardly extending edge of the floating gate and the controlgate. As shown in the prior art configuration of FIG. 1, there is afirst coupling capacitance C₁ laterally between the floating gate 1 andcontrol gate 5, and a second coupling capacitance C₂ vertically betweenthe floating gate 1 and control gate 5. The goal is to maximizetunneling efficiency while minimizing capacitive coupling between thesetwo elements. C₂ can be minimized by reducing the amount of control gate5 that overhangs floating gate 1. However, there is a constraint onminimizing C₁ as the floating gate will have a finite thickness due tofabrication process limitations. If the lateral distance between thefloating and control gates is enlarged to reduce C₁, then the insulationmaterial therebetween will be degraded by tunneling. If the lateraldistance is narrowed to enhance tunneling, then capacitive coupling C₁becomes large. Thus, capacitive coupling C₁ serves as a scaling limit.

[0064] With the present invention, however, this scaling limit isbypassed because the horizontally oriented edge 54 directly faces avertically oriented control gate spacer 56 disposed laterally adjacentthereto. There is no vertically oriented capacitive coupling (e.g. C₂),and the laterally oriented capacitive coupling (e.g. C₁) can besufficiently small while still allowing for adequate tunneling betweenthe horizontally oriented edge 54 and control gate 56.

[0065] The present invention also provides a memory cell array withreduced size and superior program efficiency. Memory cell size isreduced by as much as 50% because the bit line region 50 is buriedinside the substrate 10, and the bit line regions 50 are self aligned tothe third trenches, where space is not wasted due to limitations in thelithography generation, contact alignment and contact integrity. Cellareas of approximately 0.21 μm and 0.14 μm can be achieved by thepresent invention using 0.18 μm and 0.13 μm technology generations,respectively. Program efficiency is greatly enhanced by “aiming” thevertical portion 78 of the channel region 76 at the floating gate 14. Inconventional programming schemes, the electrons in the channel regionflow in a path parallel to the floating gate, where a relatively smallnumber of the electrons become heated and are injected onto the floatinggate. The estimated program efficiency (number of electrons injectedcompared to total number of electrons) is estimated at about 1/1000.However, because the first portion of the channel region defines anelectron path that is ‘aimed’ directly at the floating gate, the programefficiency of the present invention is estimated to be closer to 1/1,where almost all the electrons are injected onto the floating gate.

[0066] Also with the present invention, the control gates formed alongthe sidewalls of third trenches 44 can be separately optimized forconduction performance as well as punch-through immunity withoutaffecting cell size. Additionally, the punch-through suppression betweensource region 38 and the buried bit-line region 50 can be optimized byembedding the source region having a first conductivity type (e.g. Ntype) in a well having a second conductivity type (e.g. P type)different from the first conductivity type, along with using othersub-surface implant(s) that do not affect the conduction characteristicsof the memory cell. Furthermore, having source region 38 and bit-lineregion 50 separated vertically as well as horizontally allows easieroptimization of reliability parameters without affecting cell size.

[0067] Lastly, the memory cell structure of the present inventionincludes “raised source lines” 42, meaning that the conductive polyblocks 42 run along (and are electrically connected to) the source lines38, but are disposed above the substrate surface. The raised sourcelines 42 have sidewalls that are disposed laterally adjacent tosidewalls of floating gates 14, but are insulated therefrom by oxidelayers 36 and oxide spacers 40. This configuration provides decreasedelectrical resistance along the length of the source lines 38, andprovides capacitive coupling between the raised source lines 42 andfloating gates 14 (in addition to the capacitive coupling caused by theoverlap between the floating gates 14 and the source regions 38).

[0068] With the present invention, the poly blocks 42 are self alignedto the source regions 38, and the floating gates 14 are self alignedbetween the poly blocks 42 and the control gate poly spacers 56 (andthus are self aligned to the first and second portions 78/80 of thechannel regions 76).

[0069] First Alternate Embodiment

[0070]FIGS. 4A to 4G illustrate a first alternate process for formingthe memory cell structure similar to that illustrated in FIG. 3U, wherethe nitride etch used to remove nitride layer 30 is performed later inthe process to preserve part of the nitride layer. This first alternateprocess begins with the same structure as shown in FIG. 3O, which isshown again in FIG. 4A.

[0071] The formation of poly spacers 56 along sidewalls of thirdtrenches 44 is formed in the same way as described above with respect toFIG. 3P, except that a thinner poly layer is preferably deposited overthe structure (e.g. 700 Å thick), followed by a dry anisotropic dry etchprocess to remove the deposited poly layer, except for poly spacers 56left along the sidewalls of third trenches 44. The poly etch ispreferably performed to ensure the top edge of each poly spacer 56 isgenerally level with the top edge of the adjacent nitride layer 30 (asshown in FIG. 4B), or is located in-between the adjacent floating gateedge 54 and the adjacent top edge of nitride layer 30.

[0072] A nitride etch process is then used to remove nitride spacers 46.An oxide etch follows to remove oxide layers 48 and 43, and the exposedportions of oxide layer 52 at the bottom of third trenches 44(in-between poly spacers 56), as shown in FIG. 4C. Insulating spacers 58(preferably oxide) are formed along the sidewalls of third trenches 44by depositing a layer of oxide over the structure (e.g. 500 to 1200 Åthick), followed by an oxide etch (e.g. RIE). This oxide deposition andetch process also forms oxide spacers 62 adjacent the vertical portionsof nitride layer 30. The resulting structure is shown in FIG. 4D.

[0073] A metal deposition step is then performed, to deposit a metalsuch as tungsten, cobalt, titanium, nickel, platinum, or molybdenum overthe structure. The structure is then annealed, permitting the hot metalto flow and to seep into the exposed portions of the substrate 10 at thebottom of third trenches 44 to form the salicide layer 64. The hot metalalso forms the polycide layer 66 on the exposed top portions of polyspacers 56 and the polycide layer 68 on the exposed top portions of polyblocks 42. After a metal etch process to remove the remaining metal, theresulting structure is shown in FIG. 4E.

[0074] A nitride etch process follows (e.g. in hot phosphoric acid),which removes exposed portions of nitride layer 30 (i.e. those portionsnot protected by oxide spacers 62), as shown in FIG. 4F. The structureis processed as described above with respect to FIG. 3U (i.e.passivation, contact formation, and bit line formation), to result inthe structure shown in FIG. 4G.

[0075] The major difference in this first alternate embodiment from theprocess of FIGS. 3A-3U is that the nitride etch step for removingnitride layer 30 is performed later in the process. The result is thatnitride layer 30 remains intact to protect oxide layer 22 (over floatinggate 14) during the salicide/polycide metalization process. Moreover, aportion of the nitride layer 30 remains intact in the final memory cellstructure (over floating gate 14 and laterally adjacent to poly block42). Given the higher dielectric constant of nitride (compared tooxide), this remaining portion of nitride layer 30 provides a strongersidewall fringing field, and hence enhances the capacitance couplingbetween the source region 38 (including poly block 42) and the floatinggate 14 for each of the memory cells.

[0076] Second Alternate Embodiment

[0077]FIGS. 5A to 5I illustrate a second alternate process for formingthe memory cell structure similar to that illustrated in FIG. 3U, whereion implantation used to form second regions 50 is performed later inthe process, and an additional tunnel oxide formation is included. Thissecond alternate process begins with the same structure as shown in FIG.3M, which is shown again in FIG. 5A.

[0078] The same silicon and oxide etch processes are performed asdescribed above with respect to FIG. 3N to extend third trenches 44 downinto substrate 10, and remove oxide layer 48, as shown in FIG. 5B.However, the ion implantation used to form second regions 50 is notperformed at this time. Instead, the oxidation process as discussed withrespect to FIG. 3O is performed to form oxide layer 52 and floating gateedges 54, and re-grow oxide layer 48, as illustrated in FIG. 5C.

[0079] A controlled oxide etch process (e.g. with 10 to 1 diluted HFetchant) is performed to remove oxide layer 52 and expose floating gatepointed edge 54. A thin layer of oxide 82 (e.g. 130 Å) is deposited(e.g. by HTO deposition) over the structure. The removal of oxide layer52 and deposition of oxide layer 82 provides a tunneling oxide layeradjacent to the pointed edge 54 having a controlled thickness. Polyspacers 56 are then formed along sidewalls of third trenches 44 bydepositing a poly layer over the structure (e.g. 700 Å thick), followedby a dry anisotropic dry etch process to remove the deposited poly layerexcept for poly spacers 56 left along oxide layer 82 in third trenches44. Preferably, the poly etch is performed to ensure the top edge ofeach poly spacer 56 is generally level with the top edge of the adjacentnitride layer 30 (as shown in FIG. 5D), or is located in-between theadjacent floating gate edge 54 and the adjacent top edge of nitridelayer 30.

[0080] An oxide etch process is then used to remove exposed portions ofoxide layer 82 (i.e. those portions not protected by poly spacers 56)and oxide layer 48. A nitride is then used to remove nitride spacers 46,followed by an oxide etch to remove oxide layer 43. The resultingstructure is shown in FIG. 5E. Suitable ion implantation is made acrossthe entire surface of the structure to form second regions 50 (buriedbit-line regions) in the substrate 10 underneath third trenches 44.Outside of third trenches 44, the ions are blocked and have no effect.The insulating spacers 58 (preferably oxide) are formed along thesidewalls of third trenches 44 by depositing a layer of oxide over thestructure (e.g. 500 to 1200 Å thick), followed by an oxide etch (e.g.RIE). This oxide deposition and etch process also forms oxide spacers 62adjacent the vertical portions of nitride layer 30. The resultingstructure is shown in FIG. 5F. It should be noted that the ionimplantation process could be performed after the formation ofinsulating spacers 58, or even after forming control gates 56 but beforeremoval of spacers 46.

[0081] A thermal anneal process (e.g. RTA or furnace thermal anneal) isused to drive the first and second regions 38/50 deeper into thesubstrate 10. A metal deposition step is then performed, to deposit ametal such as tungsten, cobalt, titanium, nickel, platinum, ormolybdenum over the structure. The structure is then annealed to formthe salicide regions 64 (at the bottom of third trenches 44 betweenoxide spacers 58), to form the polycide regions 66 (over the exposed topportions of poly spacers 56), and to form the polycide regions 68 (onthe exposed top portions of poly blocks 42). The metal deposited on theremaining structure is removed by a metal etch process. The resultingstructure is shown in FIG. 5G.

[0082] A nitride etch process follows (e.g. in hot phosphoric acid),which removes exposed portions of nitride layer 30 (i.e. those portionsnot protected by oxide spacers 62), as shown in FIG. 5H. The structureis then processed as described above with respect to FIG. 3U (i.e.passivation, contact formation, and bit line formation), to result inthe structure shown in FIG. 51.

[0083] This second alternate embodiment shifts the ion implantation stepused to form second regions 50 until after the control gate spacers 56are formed. The oxide layer 82 used for tunneling between the floatinggate edge 54 and the control gate 56 is formed by an oxide depositionstep (e.g. HTO), instead of by a thermal growth step, to better achieveuniformity given the stress induced by the corners of third trenches 44.Lastly, the remaining portion of the nitride layer 30 provides astronger sidewall fringing field, and hence enhances the capacitancecoupling between the source region 38 (including poly block 42) and thefloating gate 14.

[0084] Third Alternate Embodiment

[0085]FIGS. 6A to 6J illustrate a third alternate process for formingthe memory cell structure similar to that illustrated in FIG. 3U, andincludes “L” shaped control gates and channel regions each having twoseparate horizontal portions separated by a vertical portion. This thirdalternate process begins with the same structure as shown in FIG. 3M,which is shown again in FIG. 6A.

[0086] The same etch process is performed as described above withrespect to FIG. 3N to extend third trenches 44 down into substrate 10,as shown in FIG. 6B. However, the ion implantation used to form secondregions 50 is not performed at this time. Instead, the oxidation processas discussed with respect to FIG. 3O is performed to form oxide layer 52and floating gate edges 54, as illustrated in FIG. 6C.

[0087] A poly layer 84 is formed over the structure. An optionalpolycide layer 86 is formed on the upper surface of poly layer 84, themetal deposition and anneal process discussed above. The total thicknessof layers 84 and 86 is preferably about 700 Å. Next, an oxide layer isformed over the structure, followed by an oxide etch that removes theoxide layer except for the oxide spacers 58 formed against poly layer 84in third trenches 44. The resulting structure is shown in FIG. 6D.

[0088] A poly etch process is used to remove the exposed portions ofpolycide layer 86 and poly layer 84 (i.e. portions not protected byoxide spacers 58), leaving “L” shaped segments of poly and polycidelayers 84/86 along the side and bottom walls of trenches 44. Preferably,this poly etch, and the formation of oxide spacers 58, are performed toensure the top edges of each poly/polycide segment 84/86 are generallylevel with the top edge of the adjacent nitride layer 30 (as shown inFIG. 6E), or is located in-between the adjacent floating gate edge 54and the adjacent top edge of nitride layer 30.

[0089] A nitride etch process is used to remove the exposed nitridespacers 46. An oxide etch is then used to remove oxide spacers 58 (inthird trenches 44), oxide layer 48 (over poly blocks 42), oxide layer43, and the portions of oxide layer 52 (at the bottom of third trenches44 between poly segments 84), as shown in FIG. 6F. An oxide depositionand etch back process (e.g. RIE dry etch) is used to form oxide spacers88 (against exposed and vertically oriented end portions of layers 52,84 and 86 in third trenches 44), oxide spacers 90 (against verticallyoriented portions of polycide layer 86 in third trenches 44), and oxidespacers 92 (against vertically oriented portions of nitride layer 30).Suitable ion implantation is made across the entire surface of thestructure to form the second regions 50 (buried bit-line regions) in thesubstrate 10 underneath third trenches 44. The ions are also implantedinto poly blocks 42 to dope (or further dope) these blocks. In areasoutside of third trenches 44, and except for poly blocks 42, the ionsare blocked and have no effect. It should be noted that the ionimplantation process could be performed before the formation of oxidespacers 88/90/92, or before the removal of nitride spacer 46. Theresulting structure is shown in FIG. 6G.

[0090] A metal deposition step is then performed, to deposit a metalsuch as tungsten, cobalt, titanium, nickel, platinum, or molybdenum overthe structure. The structure is then annealed to form the salicideregions 64 (at the bottom of third trenches 44 between oxide spacers88), the polycide regions 94 (over the exposed top portions of polylayer segments 84), and the polycide regions 68 on the exposed topportions of poly blocks 42. The metal deposited on the remainingstructure is removed by a metal etch process. The resulting structure isshown in FIG. 6H.

[0091] A nitride etch process follows (e.g. in hot phosphoric acid),which removes exposed portions of nitride layer 30 (i.e. those portionsnot protected by spacers oxide 92), as shown in FIG. 6I. The structureis then processed as described above with respect to FIG. 3U (i.e.passivation, contact formation, and bit line formation), to result inthe structure shown in FIG. 6J.

[0092] This third alternate embodiment shifts the ion implantation stepused to form second regions 50 until after the bottom of third trenches44 are narrowed by the formation of poly layer segments 84 and polycide86. Thus, second regions 50 are formed under only a center portion ofthe third trench bottom walls. This results in the formation a channelregion 76 in the substrate for each cell having three portions joinedtogether at generally right angles, with a first horizontal portion 80extending between the third trench 44 and the source region 38, avertical portion 78 extending along the vertical wall of third trench44, and a second horizontal portion 96 extending between the verticalportion 78 and the drain region 50, so that the channel region 76 isgenerally “S” shaped. The remaining portion of the nitride layer 30provides a stronger sidewall fringing field, and hence enhances thecapacitance coupling between the source region 38 (including poly block42) and the floating gate 14. Lastly, the control gate thickness isdictated by a poly deposition step, as opposed to poly deposition andetch back process that can be more difficult to precisely control.

[0093] Fourth Alternate Embodiment

[0094]FIGS. 7A to 7Z illustrate a fourth alternate process for formingthe memory cell structure similar to that illustrated in FIG. 3U, bututilizing a self alignment contact (SAC) scheme. This fourth alternateprocess begins with the same structure as shown in FIG. 3C, which isshown again in FIG. 7A.

[0095] A thick layer of dielectric material 102 (e.g. BSG) is depositedover the structure, as shown in FIG. 7B, to fill gaps between portionsof oxide layer 32. A BSG etch process is used to etch down the BSG layer102 generally even with the tops of oxide layer 32, using oxide layer 32as an etch stop. The BSG etch process results in blocks of the BSG 102disposed between the tops of oxide layer 32, as shown in FIG. 7C. Anoxide etch process is used to etch the exposed portions of oxide layer32 down generally even with the tops of nitride layer 30, using nitridelayer 30 as an etch stop. A controlled oxide over-etch is used to etchdown the exposed portion of oxide layer 32 a predetermined distancebelow the tops of nitride layer 30, as shown in FIG. 7D.

[0096] Next, a thick nitride layer 104 is deposited over the structure,as shown in FIG. 7E. A planarization etch process follows (e.g. CMP) toetch down nitride layer 104, BSG 102 and nitride 30 even with the topsof poly blocks 28 (using poly blocks 28 as the polishing stopper), andto leave the top surfaces of poly blocks 28 exposed. The resultingstructure is shown in FIG. 7F.

[0097] A poly etch process is used to remove poly blocks 28 (using oxidelayer 22 as an etch stop) to form second trenches 34. A controlled oxideetch is used to remove the portions of oxide layer 22 exposed at thebottoms of second trenches 34 (using poly layer 14 as an etch stop).Another poly etch process is performed to remove portions of poly layer14 exposed at the bottoms of the second trenches 34 (using oxide layer12 as an etch stop). The resulting structure is shown in FIG. 7G.

[0098] An oxide sidewall layer 36 is formed on the exposed ends of polylayer 14 at trenches 34 using a thermal oxidation process. Suitable ionimplantation is then used to form first regions (source regions) 38 inthe substrate 10 having a conductivity type (e.g. N type) that isdifferent from that of the surrounding substrate (e.g. P type). Sidewallspacers 40 are then formed against the walls of the second trenches 34by depositing a thin layer of oxide (e.g. 200 Å) over the entirestructure, followed by an anisotropic etch process (e.g. RIE dry etch),to remove the deposited oxide layer except for spacers 40. This oxideetch process also removes the exposed portions of oxide layer 12 at thebottom of second trenches 34 to expose the substrate 10. The resultingstructure is shown in FIG. 7H.

[0099] The second trenches are filled with poly blocks 42, preferably bydepositing a thick layer of poly silicon over the structure, followed bya planarization process (i.e. CMP) to remove the deposited poly siliconexcept for poly blocks 42 in second trenches 34. An optional poly etchis be performed to slightly recess the upper surfaces of poly blocks 42relative to nitride layer 104 and oxide layer 32, to address anytopography issues if present. The poly blocks 42 are in directelectrical contact with source regions 38 of substrate 10, and can bedoped in-situ or by a separate implant. The resulting structure is shownin FIG. 7I.

[0100] A BSG etch process (wet or dry) is used to remove BSG 102,exposing portions of oxide layer 32. An anisotropic etch (e.g. RIE) isthen used to remove exposed portions of oxide layer 32 (i.e. portionsnot protected by nitride 104), leaving semi-recessed third trenches 44disposed between oxide blocks 32, as illustrated in FIG. 7J.

[0101] Next, the structure is subjected to a thermal oxidation process,which forms an oxide layer 48 over poly blocks 42. Oxide layer 48 isself aligned to the poly blocks 42 (with a thickness, for example, of˜600 Å). During the thermal cycle of this oxidation process, the sourceregions 38 are driven deeper into the substrate. The resulting structureis shown in FIG. 7K.

[0102] An anisotropic (dry) nitride etch is used to remove the portionsof nitride layer 30 that are exposed in third trenches (between oxideblocks 32), using oxide layer 22 as an etch stop. This nitride etch alsoremoves some of the exposed portions of nitride layer 30 adjacent polyblocks 42, as well as reducing the thickness of nitride layer 104 (e.g.down to a thickness of ˜300-500 Å). An anisotropic oxide etch follows toremove the portions of oxide layer 22 that are exposed in third trenches44 (between oxide blocks 32), using poly layer 14 as an etch stop. Thisoxide etch also slightly consumes small portions of oxide layer 48. Theresulting structure is shown in FIG. 7L.

[0103] An anisotropic (dry) poly etch is next performed to remove theportions of poly layer 14 that are exposed in third trenches (betweenoxide blocks 32), using oxide layer 12 as an etch stop. This poly etchis preferably performed to form a slope or taper region 106 on the edgeof poly layer 14 facing third trenches 44, as shown in FIG. 7M.

[0104] An anisotropic (dry) oxide etch follows, which removes theportions of oxide layer 12 that are exposed in third trenches betweenoxide blocks 32), which exposes the substrate 10 at the bottom of thirdtrenches 44. This oxide etch also slightly consumes small portions ofoxide layer 48. The resulting structure is shown in FIG. 7M. Next, withthe silicon substrate 10 left exposed at the bottom of third trenches44, a silicon (dry) etch process is performed to extend third trenches44 down into substrate 10, to a depth of 500 Å to 1500 Å below thesubstrate surface. This etch is selected to have a 1-to-1 selectivitybetween silicon and oxide, so that it also etches a similar depth trenchin the oxide isolation regions 16 (LOCOS or STI), wherein third trenches44 are continuously formed and extend across the active and isolationregions 17/16. The resulting structure is shown in FIG. 7N.

[0105] An oxidation process is then performed, which oxidizes theexposed silicon surfaces of third trenches 44 to form a thin layer ofoxide 52 (e.g. 150 Å) lining these silicon surfaces. This oxidationprocess also oxidizes the exposed tapered sides 106 of poly layer 14that form part of the sidewalls of upper portions of third trenches 44,which results in horizontally oriented edges 54 each of which pointsdirectly toward one of the sidewalls of third trenches 44, asillustrated in FIG. 7O. Edges 54 can be elongated edges (i.e. like thesharp edge of a razor blade), or shorter edges (i.e. like the tip of apencil). This oxidation process also thickens the oxide layer 48 overpoly blocks 42.

[0106] An optional oxide layer optimization process is next, where thethickness of the thermally grown Fowler-Nordheim tunneling oxide can beoptimized by removing oxide layer 52 from the third trenches 44,followed by an HTO oxide deposition step to reform oxide layer 52 overthe entire structure having a well controlled thickness (e.g. 150 Å).The resulting structure is shown in FIG. 7P.

[0107] A poly layer 56 is formed over the structure (e.g. 800 Å thick),as shown in FIG. 7Q. Metalized polysilicon (not shown) can be formed onthe upper surface of poly layer 56. A layer of dielectric material 108(e.g. BSG, ˜1000 Å) is then formed over poly layer 56, as shown in FIG.7R. A BSG etch process is then used to etch BSG 108 down even with oxidelayer 52, followed by a poly etch to etch poly layer 56 down even withoxide layer 52. A CMP planarization could alternately be used instead ofthese BSG and poly etches. An additional poly etch is performed torecess the tops of poly layer 56 below the tops of oxide layer 52 andBSG 108, as illustrated in FIG. 7S.

[0108] A nitride layer 110 is next deposited over the structure (e.g.800 Å), as shown in FIG. 7T. A nitride etch follows, using oxide layer52 as an etch stop, which leaves nitride blocks 110 over the poly layer56, as shown in FIG. 7U. Nitride blocks 110 are self aligned to the polylayer 56 by oxide layer 52 (and oxide block 32) and BSG 108. A BSG etchis then used to remove BSG blocks 108 from the third trenches 44. Theresulting structure is shown in FIG. 7V.

[0109] An anisotropic poly etch is used to remove the exposed portionsof poly 56 at the bottom of third trenches 44 (i.e. those portions notprotected by nitride 110), as shown in FIG. 7W. Next, nitride isdeposited over the structure (e.g. 300 Å thick), followed by ananisotropic nitride etch (e.g. RIE) that removes the deposited nitrideexcept for nitride spacers 112 in third trenches 44. Suitable ionimplantation is once again made across the entire surface of thestructure to form second regions 50 (buried bit-line drain regions) inthe substrate 10 underneath third trenches 44. Outside of third trenches44, the ions are blocked and have no effect. It should be noted that theion implantation process could be performed after the oxide etchperformed next. The resulting structure is shown in FIG. 7X.

[0110] An oxide etch step is next, which removes the exposed portions ofoxide layer 52 and oxide layer 48. A metal deposition step is thenperformed, to deposit a metal such as tungsten, cobalt, titanium,nickel, platinum, or molybdenum over the structure. The structure isthen annealed to form a conductive layer of metalized silicon 64(silicide) in the exposed substrate between oxide spacers 112, and aconductive layer of metalized polysilicon 66 (polycide) on the exposedtop portions of poly blocks 42. The metal deposited on the remainingstructure is removed by a metal etch process. The resulting structure isshown in FIG. 7Y.

[0111] An oxide layer 114 (e.g. BP TEOS), is used to cover thestructure. A masking step is performed to define etching areas over thesalicide regions 64. The oxide layer 114 is selectively etched in theetching areas to create contact openings that are ideally centered overthe salicide regions 64 (and second regions 50). The contact openingsare then filled with conductor metal by metal deposition and planarizingetch-back, forming conductor contacts 72. The salicide layer 64 enhancesconduction between the conductors 72 and the second regions 50. Polycidelayer 66 enhances conduction along the length of poly blocks 42. A bitline connector 74 is added to each active region by metal masking overthe oxide 114 to connect together all the contacts 72 in that activeregion. The final structure is shown in FIG. 7Z.

[0112] The formation of contact 72 is referred to as a self alignedcontact scheme (SAC) because the width of the contact is made wider thanthe separation between adjacent, facing nitride spacers 112, and thus isself aligned to the drain region 50. Part of the contact 72 is formeddirectly over poly block 56, but is insulated therefrom by nitride layer110, thus ensuring that a good contact with the drain region 50 isachieved.

[0113] The self aligned contact scheme (SAC) removes an importantconstraint on the minimum spacing requirement between adjacent sets ofpaired memory cells. Specifically, while FIG. 7Z illustrates the contactarea (and thus conductors 72) perfectly centered over the drain regions50, in reality it is very difficult to form the contact openings withoutsome undesirable horizontal shift relative to the drain regions 50. Afaulty connection could occur if the horizontal shift becomes greatenough to prevent contact 72 from completely filling in the spacebetween spacers 112. With a non-self aligned contact scheme, such asthat used with the embodiment illustrated in FIG. 3U, where there is noprotective layer of nitride over poly spacers 56, electrical shorts canoccur if the contact 72 is shifted over and makes contact with polyspacers 56. To prevent electrical shorts in the non-self aligned contactscheme, the contact openings are formed with edges sufficiently awayfrom the spacers 58 so that even with the maximum possible shift in thecontact regions, they will not extend beyond the spacers 58. This ofcourse presents a constraint on the minimum distance between spacers 58for the embodiment shown in FIG. 3U, in order to provide a sufficienttolerance distance between adjacent sets of paired mirror cells.

[0114] SAC as used in the fourth alternate embodiment eliminates thisconstraint by using a protective layer of material (nitride layer 110)over poly blocks 56. With this protective layer, the contact openingsare allowed to be formed in the oxide layer 114 with a sufficient widthto ensure there is overlap of the contact opening with the drain regions50, even if there is a significant horizontal shift of the contactopening during formation. Nitrite layer 110 allows portions of contact72 to be formed over poly blocks 56 without any shorting therebetween.Thus, the width of contact regions between spacers 72 can be minimized,allowing the scaling down of the overall cell dimension. It should benoted that SAC can be utilized with any of the method embodimentsillustrated in this application.

[0115] As with the third alternative embodiment, this fourth alternateembodiment results in second regions 50 being formed under only a centerportion of the third trench bottom walls, and thus “S” shaped channelregions 76 have three portions joined together at generally rightangles: first horizontal portion 80 extending between the third trench44 and the source region 38, vertical portion 78 extending along thevertical wall of third trench 44, and second horizontal portion 96extending between the vertical portion 78 and the drain region 50. Thegenerally rectangular control gates 56 each have a first portiondirectly adjacent the channel region vertical portion 78 and a secondportion directly adjacent the channel region second horizontal portion96. The remaining portion of the nitride layer 30 provides a strongersidewall fringing field, and hence enhances the capacitance couplingbetween the source region 38 (including poly block 42) and the floatinggate 14. The floating gates 14 have the horizontally oriented edge 54that directly faces the vertically oriented control gate blocks 56disposed laterally adjacent thereto. Lastly, the control gate oxidethickness is dictated by a poly deposition step, as opposed to polydeposition and etch back process that can be more difficult to preciselycontrol.

[0116] Fifth Alternate Embodiment

[0117]FIGS. 8A to 8W illustrate a fifth alternate process for formingthe memory cell structure similar to that illustrated in FIG. 7Z, butwith a metal source line scheme. This fifth alternate process beginswith the same structure as shown in FIG. 7F.

[0118] A BSG etch (e.g. wet etch) is used to remove BSG 102 as shown inFIG. 8A, followed by an anisotropic oxide etch (e.g. RIE) that removesexposed portions of oxide layer 32 (i.e. portions not protected bynitride 104), leaving semi-recessed third trenches 44 disposed betweenoxide blocks 32. Next, the structure is subjected to a thermal oxidationprocess, which forms an oxide layer 48 over poly blocks 28. Oxide layer48 is self aligned to the poly blocks 28 (with a thickness, for example,of ˜600 Å). The resulting structure is shown in FIG. 8B.

[0119] An anisotropic (dry) nitride etch is used to remove the portionsof nitride layer 30 that are exposed in third trenches 44 (between oxideblocks 32), using oxide layer 22 as an etch stop. This nitride etch alsoremoves some of nitride layer 104 (e.g. leaving a thickness of ˜500 Å),as well as the exposed portions of nitride layer 30 adjacent poly blocks28. An anisotropic oxide etch follows to remove the portions of oxidelayer 22 that are exposed in third trenches 44 (between oxide blocks32), using poly layer 14 as an etch stop. This oxide etch also slightlyconsumes small portions of oxide layer 48. The resulting structure isshown in FIG. 8C.

[0120] An anisotropic (dry) poly etch is next performed to remove theportions of poly layer 14 that are exposed in third trenches (betweenoxide blocks 32), using oxide layer 12 as an etch stop. This poly etchis preferably performed to form a slope or taper region 106 on the edgeof poly layer 14 facing third trenches 44, as shown in FIG. 8D.

[0121] An anisotropic (dry) oxide etch follows, which removes theportions of oxide layer 12 that are exposed in third trenches 44(between oxide blocks 32), which exposes the substrate 10 at the bottomof third trenches 44. This oxide etch also slightly consumes smallportions of oxide layer 48. With the silicon substrate 10 left exposedat the bottom of third trenches 44, a silicon (dry) etch process isperformed to extend third trenches 44 down into substrate 10, to a depthof 500 Å to 1500 Å below the substrate surface. This etch is selected tohave a 1-to-1 selectivity between silicon and oxide, so that it alsoetches a similar depth trench in the oxide isolation regions 16 (LOCOSor STI), wherein third trenches 44 are continuously formed and extendacross the active and isolation regions 17/16. The resulting structureis shown in FIG. 8E.

[0122] An oxidation process is then performed, which oxidizes theexposed silicon surfaces of third trenches 44 to form a thin layer ofoxide 52 (e.g. 150 Å) lining these silicon surfaces. This oxidationprocess also oxidizes the exposed tapered sides 106 of poly layer 14that form part of the sidewalls of upper portions of third trenches 44,which results in horizontally oriented edges 54 each of which pointsdirectly toward one of the sidewalls of third trenches 44, asillustrated in FIG. 8F. Edges 54 can be elongated edges (i.e. like thesharp edge of a razor blade), or shorter edges (i.e. like the tip of apencil). This oxidation process also thickens the oxide layer 48 overpoly blocks 42.

[0123] An optional oxide layer optimization process is next, where thethickness of the thermally grown Fowler-Nordheim tunneling oxide can beoptimized by removing oxide layer 52 from the third trenches 44,followed by an HTO oxide deposition step to reform oxide layer 52 overthe entire structure having a well controlled thickness (e.g. 150 Å).The resulting structure is shown in FIG. 8G.

[0124] A poly layer 56 is formed over the structure (e.g. 800 Å thick),as shown in FIG. 8H. Metalized polysilicon (not shown) can be formed onthe upper surface of poly layer 56. A layer of dielectric material 108(e.g. BSG, ˜1000 Å) is then formed over poly layer 56, as shown in FIG.81. A BSG etch process is then used to etch BSG 108 down even with oxidelayer 52, followed by a poly etch to etch poly layer 56 down even withoxide layer 52. A CMP planarization could alternately be used instead ofthese BSG and poly etches. An additional poly etch is performed torecess the tops of poly layer 56 below the tops of oxide layer 52 andBSG 108, as illustrated in FIG. 8J.

[0125] A nitride layer 110 is next deposited over the structure (e.g.800 Å), as shown in FIG. 8K. A nitride etch follows, using oxide layer52 as an etch stop, which leaves nitride blocks 110 over the poly layer56, as shown in FIG. 8L. Nitride blocks 110 are self aligned to the polylayer 56 by oxide layer 52 (and oxide block 32) and BSG 108. A BSG etchis then used to remove BSG blocks 108 from the third trenches 44. Theresulting structure is shown in FIG. 8M.

[0126] An anisotropic poly etch is used to remove the exposed portionsof poly 56 at the bottom of third trenches 44 (i.e. those portions notprotected by nitride 110, as shown in FIG. 8N. Next, nitride isdeposited over the structure (e.g. 300 Å thick), followed by ananisotropic nitride etch (e.g. RIE) that removes the deposited nitrideexcept for nitride spacers 112 in third trenches 44, as illustrated inFIG. 8O.

[0127] Suitable ion implantation is made across the entire surface ofthe structure to form second regions 50 (buried bit-line drain regions)in the substrate 10 underneath third trenches 44. Outside of thirdtrenches 44, the ions are blocked and have no effect. It should be notedthat this ion implantation process could be performed at an earlier or alater step in the process. The resulting structure is shown in FIG. 8P.

[0128] An oxide layer 116 is deposited over the structure, which fillsthird trenches, as shown in FIG. 8Q. The top surface of the structure isplanarized (e.g. CMP process) using nitride layer 110 as an etch stopand exposing poly blocks 28, as shown in FIG. 8R.

[0129] A poly etch process is used to remove poly blocks 28 (using oxidelayer 22 as an etch stop) to form second trenches 34. A controlled oxideetch (e.g. HF) is used to remove the portions of oxide layer 22 exposedat the bottoms of second trenches 34 (using poly layer 14 as an etchstop). Another poly etch process is performed to remove portions of polylayer 14 exposed at the bottoms of the second trenches 34 (using oxidelayer 12 as an etch stop). Suitable ion implantation is then used toform first regions (source regions) 38 in the substrate 10 having aconductivity type (e.g. N type) that is different from that of thesurrounding substrate (e.g. P type). The resulting structure is shown inFIG. 8S.

[0130] An oxide sidewall layer 36 is formed on the exposed ends of polylayer 14 at trenches 34 using a thermal oxidation process, as shown inFIG. 8T. Sidewall spacers 40 are then formed against the walls of thesecond trenches 34 by depositing a thin layer of oxide (e.g. 200 Å) overthe entire structure, followed by an anisotropic etch process (e.g. RIEdry etch), to remove the deposited oxide layer except for spacers 40.This oxide etch process also removes the exposed portions of oxide layer12 at the bottom of second trenches 34 to expose the substrate 10, aswell as consumes some of oxide layer 52. The resulting structure isshown in FIG. 8U. The second trenches 34 are then filled with blocks 120of metal material in the following manner. A layer of TiN material 118is preferably deposited over the structure, followed by the depositionof a thick layer of conductive metal such as aluminum or tungsten. Then,a metal planarization step follows (e.g. CMP), which etches the metallayer down even with the tops of the second trenches 34, leaving blocks120 of conductive metal in second trenches 34 and in electrical contactwith the source regions 38 via TiN 118. An optional metal recess etchcan be performed to ensure all the metal deposited outside of secondtrenches 34 has been removed. The resulting structure is shown in FIG.8V.

[0131] An oxide layer 114 (e.g. BP TEOS), is used to cover thestructure. A masking step is performed to define etching areas over thedrain regions 50. The oxide layers 114, 116, and 52 are selectivelyetched from the etching areas to create contact openings that areideally centered over and significantly wider than the exposed portionsof second region 50. The contact openings are then filled with conductormetal by metal deposition and planarizing etch-back, forming conductorcontacts 72. A bit line connector 74 is added to each active region bymetal masking over the oxide 114 to connect together all the contacts 72in that active region. The final structure is shown in FIG. 8W.

[0132] In addition to the benefits of a horizontally oriented floatinggate sharp edge directed toward a vertically oriented control gate, SACalignment of the metal contacts 72, “S” shaped channel region, and thecontrol gate length being dictated by a poly deposition step, thisembodiment has the further advantage of forming a source line 38 with ablock of metallic material attached along its length, thus reducingoverall resistance of the source line 38 over its length.

[0133] It is to be understood that the present invention is not limitedto the embodiments described above and illustrated herein, butencompasses any and all variations falling within the scope of theappended claims. For example, third trenches 44 can end up having anyshape that extends into the substrate, not just the elongatedrectangular shape shown in the figures. Also, although the foregoingmethod describes the use of appropriately doped polysilicon as theconductive material used to form the memory cells, it should be clear tothose having ordinary skill in the art that any appropriate conductivematerial can be used. In addition, any appropriate insulator can be usedin place of silicon dioxide or silicon nitride. Moreover, anyappropriate material whose etch property differs from silicon dioxide(or any insulator) and from polysilicon (or any conductor) can be usedin place of silicon nitride. Further, as is apparent from the claims,not all method steps need be performed in the exact order illustrated orclaimed, but rather in any order that allows the proper formation of thememory cell of the present invention. Additionally, the above describedinvention is shown to be formed in a substrate which is shown to beuniformly doped, but it is well known and contemplated by the presentinvention that memory cell elements can be formed in well regions of thesubstrate too, which are regions that are doped to have a differentconductivity type compared to other portions of the substrate. Lastly,single layers of insulating or conductive material could be formed asmultiple layers of such materials, and vice versa.

1-44. (Cancelled).
 45. A method of forming a semiconductor memory cell,comprising the steps of: forming first and second spaced apart regionsin a semiconductor substrate having a conductivity type different fromthat of the substrate, wherein a channel region of the substrate isdefined between the first and second regions; forming a floating gate ofelectrically conductive material disposed over and insulated from atleast a portion of the channel region, wherein the floating gateincludes a horizontally oriented edge extending from a lateral side ofthe floating gate; and forming a control gate of electrically conductivematerial having at least a portion thereof disposed laterally adjacentto and insulated from the horizontally oriented edge.
 46. The method ofclaim 45, wherein the floating gate is formed to be disposed over andinsulated from a portion of the second region.
 47. The method of claim45, further comprising the step of: forming a trench into a surface ofthe semiconductor substrate and spaced apart from the first region,wherein the second region is formed underneath the trench.
 48. Themethod of claim 47, wherein the channel region includes a first portionthat extends generally along a sidewall of the trench and a secondportion that extends generally along the substrate surface.
 49. Themethod of claim 48, wherein the first and second portions of the channelregion extend in directions that are generally perpendicular to eachother.
 50. The method of claim 47, wherein at least a portion of thecontrol gate is formed to extend into the trench.
 51. The method ofclaim 47, wherein: the floating gate is generally elongated and extendsin a direction generally parallel to the substrate surface; and thecontrol gate is generally elongated and extends in a direction generallyperpendicular to the substrate surface.
 52. The method of claim 47,wherein the formation of the control gate includes forming a spacer ofthe electrically conductive material having a first portion extendingalong and insulated from a sidewall of the trench and a second portiondisposed laterally adjacent to and insulated from the horizontallyoriented edge.
 53. The method of claim 45, further comprising the stepof: forming a block of conductive material disposed over and inelectrical contact with the first region.
 54. The method of claim 53,wherein the floating gate is disposed laterally adjacent to andinsulated from the block of conductive material.
 55. The method of claim48, wherein the floating gate is formed generally over and insulatedfrom the entire second portion of the channel region.
 56. The method ofclaim 45, further comprising the step of: forming a layer of insulatingmaterial between the floating gate edge and the control gate having athickness permitting Fowler-Nordheim tunneling of charges therethrough.57. The method of claim 48, wherein the channel region first portionextends in a direction directly toward the floating gate.
 58. The methodof claim 54, further comprising the step of: forming a layer ofinsulating material disposed over the floating gate and laterallyadjacent to the block of conductive material.
 59. The method of claim58, wherein the layer of insulating material is made of silicon nitride.60. The method of claim 48, wherein the formation of the control gateincludes the steps of: forming a first portion of the control gate thatextends along and is insulated from a sidewall of the trench; andforming a second portion of the control gate that extends along and isinsulated from a bottom wall of the trench.
 61. The method of claim 60,wherein the control gate is generally “L” shaped.
 62. The method ofclaim 60, wherein the control gate is generally rectangular shaped. 63.The method of claim 60, wherein the channel region includes a thirdportion that extends generally along the bottom wall of the trench. 64.The method of claim 63, wherein the channel region is generally “S”shaped.
 65. The method of claim 45, further comprising the step of:forming a metal contact having a first portion disposed over andelectrically connected to the second region and a second portiondisposed over and insulated from the control gate.
 66. The method ofclaim 53, wherein the block conductive material is a metal.
 67. A methodof forming an array of semiconductor memory cells, comprising the stepsof: forming spaced apart isolation regions on the substrate having afirst conductivity type which are generally parallel to one another andextend in a first direction, with an active region between each pair ofadjacent isolation regions; forming a plurality of spaced apart firstand second regions in the semiconductor substrate having a secondconductivity type, wherein a plurality of channel regions in the activeregions of the substrate are defined each extending between one of thefirst regions and one of the second regions; forming a plurality offloating gates of electrically conductive material each disposed overand insulated from at least a portion of one of the channel regions,wherein each of the floating gates includes a horizontally oriented edgeextending from a lateral side of the floating gate; and forming aplurality of electrically conductive control gates each having at leasta portion thereof disposed laterally adjacent to and insulated from oneof the horizontally oriented edges.
 68. The method of claim 67, whereinthe plurality of control gates are generally parallel to one another andextend in a second direction generally perpendicular to the firstdirection across the active and isolation regions.
 69. The method ofclaim 67, wherein each of the floating gates are disposed over andinsulated from a portion of one of the first regions.
 70. The method ofclaim 67, further comprising the step of: forming a plurality oftrenches into a surface of the semiconductor substrate that aregenerally parallel to one another and extend in a second directiongenerally perpendicular to the first direction across the active andisolation regions, wherein each of the second regions is formedunderneath one of the trenches.
 71. The method of claim 70, wherein eachof the channel regions includes a first portion that extends generallyalong a sidewall of one of the trenches and a second portion thatextends generally along the substrate surface.
 72. The method of claim71, wherein the first and second portions of the channel region extendin directions that are generally perpendicular to each other.
 73. Themethod of claim 71, wherein at least a portion of each of the controlgates is formed to extend into one of the trenches.
 74. The method ofclaim 71, wherein the formation of each of the control gates includesforming a spacer of the electrically conductive material having a firstportion extending along and insulated from a sidewall of one of thetrenches and a second portion disposed laterally adjacent to andinsulated from one of the horizontally oriented edges.
 75. The method ofclaim 70, wherein: each of the floating gates is generally elongated andextends in a direction generally parallel to the substrate surface; andeach of the control gates is generally elongated and extends in adirection generally perpendicular to the substrate surface.
 76. Themethod of claim 67, further comprising the step of: forming a pluralityof blocks of conductive material that are generally parallel to oneanother and extend in a second direction generally perpendicular to thefirst direction across the active and isolation regions, wherein each ofthe conductive material blocks is disposed over and in electricalcontact with some of the first regions.
 77. The method of claim 76,wherein each of the floating gates is disposed laterally adjacent to andinsulated from one of the blocks of conductive material.
 78. The methodof claim 71, wherein each of the floating gates is formed generally overand insulated from the entire second portion of one of the channelregions.
 79. The method of claim 67, further comprising the step of:forming insulating material between each of the floating gate edges andthe adjacent control gate having a thickness permitting Fowler-Nordheimtunneling of charges therethrough.
 80. The method of claim 71, whereineach of the channel region first portions extends in a directiondirectly toward one of the floating gates.
 81. The method of claim 77,further comprising the step of: forming a layer of insulating materialdisposed over each of the floating gates and laterally adjacent to oneof the blocks of conductive material.
 82. The method of claim 81,wherein the layer of insulating material is made of silicon nitride. 83.The method of claim 71, wherein the formation of each of the controlgates includes the steps of: forming a first portion of the control gatethat extends along and is insulated from a sidewall of one of thetrenches; and forming a second portion of the control gate that extendsalong and is insulated from a bottom wall of the one trench.
 84. Themethod of claim 83, wherein each of the control gates is generally “L”shaped.
 85. The method of claim 83, wherein each of the control gates isgenerally rectangular shaped.
 86. The method of claim 83, wherein eachof the channel regions includes a third portion that extends generallyalong the bottom wall of the one trench.
 87. The method of claim 86,wherein each of the channel regions is generally “S” shaped.
 88. Themethod of claim 67, further comprising the step of: forming a pluralityof metal contacts each having a first portion disposed over andelectrically connected to one of the second regions and a second portiondisposed over and insulated from one of the control gates.
 89. Themethod of claim 76, wherein the block conductive material is a metal.90-92. (Cancelled).